Many techniques for analyzing whole blood samples have been proposed in the prior art. In some prior art methods, the fluid components are separated from the red blood cells (hematocrit) by first filtering the whole blood sample through a porous medium, such as a layer of cellulose. The red blood cells are trapped on top of the filter. The fluid components passing through the filter are then deposited upon a tape or other medium containing a reagent for testing the fluid sample for a particular component such as: glucose, BUN, Na.sup.+, etc. Such techniques may be seen with reference to U.S. Pat. Nos. 3,260,413, issued July 12, 1966, and 3,261,668, issued July 19, 1966.
In the above methods, the object of the invention is to perform rapid analyses of the blood samples in an automated fashion, using a minimum of sample volume, e.g., 20 microliters. While such techniques are generally useful for their intended purposes, they are not totally successful. This is because each filtered fluid sample must be carefully metered to obtain an aliquot. When dealing with small quantities of fluid such as 20 microliters, even a small error in sample volume will give a totally erroneous result.
It has been realized, therefore, that a major problem in the automated analysis of whole blood could be eliminated by obtaining an accurate aliquot of sample in a simple and convenient manner.
In the U.S. patent application to Martin J. Lee, Ser. No. 922,611, filed July 7, 1978, assigned to a common assignee, it has been proposed that an aliquot of a blood analyte could be obtained by diffusion of the blood analyte from a serum sample into a gel or other porous medium for a given period of time. This approach, therefore, suggests a measurement of time in contrast to previous sample volume metering. Accurate time measurements are easier to achieve than sample volume measurements. However, if whole blood is used, the diffusion of fluid components are dependent upon factors other than time. For example, a correction factor is needed to account for changes in diffusion rates resulting from variations in hematocrit from sample to sample, i.e., each whole blood sample has a different amount of red blood cells which alters the diffusion rate of fluid diffusing into the porous medium. (See U.S. patent application to Martin J. Lee and Michael J. Malin, Ser. No. 936,436, filed Aug. 24, 1978, now U.S. Pat. No. 4,250,257, assigned to a common assignee.)
In one embodiment of the present invention, this previous problem is overcome by obtaining an accurate aliquot using a method which is neither time nor hematocrit dependent. The invention seeks to provide a thin film of gel or other porous material having a known predetermined volume. When a drop of whole blood is deposited on the gel, it will equilibrate or otherwise diffuse to saturation almost immediately, due to the minute volume and thin-layer geometry of the gel. In other words, rather than meter the sample to obtain an aliquot, this embodiment of the invention controls the volume of the receptacle into which the sample is diffused. The manufacture of precise gel volumes can be easily controlled, thus providing for automation of whole blood sample analyses which are accurate, rapid, and convenient.
In other embodiments of the invention, a timed diffusion is more accurately achieved than disclosed by the prior art, through the use of a molecular diffusion switch or valve. The molecular diffusion switch comprises an impermeable barrier disposed between two porous media across which diffusion is to occur. The impermeable barrier is removed and then restored, such that the diffusion across the media is precisely time controlled. In a preferred arrangement, the impermeable barrier is an immiscible fluid interposed between the two porous media containing diffusable species. The impermeable barrier fluid is easily displaced from between the media and then restored therebetween, to achieve the objective of controlling precisely the initiation and duration time of diffusion between the two porous media to provide an accurate aliquot.